Web feed mechanisms including tractor mechanisms have been used for many years for transporting webs through printers and other similar apparatus. Typically, the paper utilized has pre-punched holes or perforations along the edges thereof. Usually at least two drive mechanisms are mounted to engage opposite edges of the web and they include pins which extend through the perforations in the web to positively engage and drive the web. The drive pins can extend outwardly from a driven endless loop or belt, or they may extend radially from the perimeter of a wheel or roller. In either case, it is essential that the web and its associated perforations be guided onto and snugly seated against the drive pins.
The requirement of guiding the web and snugly seating it against the drive pins is critical to the proper operation of the web transport mechanism. From a mechanical standpoint, the web must be started, moved, and stopped, and often at high speeds. If the web perforations are free of the drive pins, there will be no movement at all. If the web perforations are loosely seated on the drive pins such that they may move relative to the pins, the movement will tear or destroy the web, resulting in jamming of the web transport mechanism, ripping of the edge perforations, and undesirable wrinkling of the web. This may even result in damage to the ribbon, platen, print head, or other parts of the printer. In addition to the resulting damage to the web or printer, there is down time resulting in a loss of productivity, and the necessity of replacing the web and reprinting all of the information.
This problem is aggravated by the delicate nature of the web, which is typically made of a thin sheet of paper, and the delicate edge perforations, which are also typically made of paper and designed to easily detach from the body of the web. Unless the web perforations are snugly seated onto the base of the drive pin and maintained in such a position, they have a tendency to rise off the pin as the web advances through the mechanism. Since the drive mechanism usually steps the web through the printer in a series of rapid starts and stops, and may even reverse the direction of feed, the force of the pin against the perforation will likely damage the web unless it is properly seated on the pins.
In the past, one of the solutions to this problem was the use of a paper guide or cover which loosely retained the web in engagement with the pins to inhibit accidental displacement. See U.S. Pat. No. 2,277,156 to Sherman et al. Another solution is presented in U.S. Pat. No. 4,226,353 to Blaskovic et al, which discloses a door for a tractor drive that has its bottom surface parallel to the top driving surface of the tractor with the web sandwiched therebetween. However, these solutions suffered because of the competing considerations requiring that the door be close enough to the driving surface to press the web onto the pins, yet spaced enough to prevent the friction from the door and drive surface from unduly restraining movement of the web. These opposing requirements become more acute when the same web advance mechanism is offered to handle thin single sheet webs and thicker multi-layer forms. One solution to this problem was to make the spacing between the door and driving surface adjustable, to accommodate webs of varying thickness. However, this requires constant attention and readjustment. See U.S. Pat. No. 3,688,959 to Staneck et al.
Still other proposed solutions to this dilemma involved the use of active elements, such as rollers or belts that firmly pressed the web against the drive
In U.S. Pat. Nos. 2,730,358 to Riordan and 3,439,852 to Blodgett a multi-roller mechanism compresses the web, and in U.S. Pat. Nos. 3,608,801 to Nystrand and 3,669,327 to Dowd an active belt and system is employed. These four patents epitomize the complexity and large number of parts required in active systems, yet demonstrate the importance of keeping the web firmly seated against the drive pins while minimizing the friction retarding movement of the web.
Accordingly, it is an object of the present invention to provide a web feed mechanism and guide for urging the web and its associated perforations down onto the drive pins.
It is a further object of the present invention to provide a web feed mechanism that urges a web and its associated perforations down onto the drive pins without imparting undue friction on the web.
It is a still further object of the present invention to provide web feed mechanism having a means for guiding, where the means for guiding includes static protrusions to guide the web onto the drive pins.
It is a still further object of the present invention to provide a web feed mechanism for guiding the web onto the drive pins that eliminates the need for complex mechanical systems and adjustment mechanisms.
These and other objects are accomplished by generally providing a feed mechanism moving a web, as disclosed and claimed herein. It includes a means for engaging a web at uniformly spaced perforations with drive pins that extend through the web perforations. A means for guiding has a surface below the upper ends of the pins and is generally aligned with the pins. The means for guiding has at least one static protrusion proximate a pin insertion portion of the feed mechanism, with the protrusion extending below the surface of the means for guiding to urge the web and its associated perforations down onto the drive pins.